The R-21 (Russian: Р-21; NATO: SS-N-5 'Sark/Serb'; GRAU: 4K55) was a submarine-launched ballistic missile in service with the Soviet Union between 1963 and 1989.

Development of the R-15 and R-21 was initially assigned to Mikhail Yangel's Yuzhnoye Design Office (OKB-586) on 20 March 1958.

It was intended for the Golf-class submarines (Project 629B); the complete missile system was called the D-4 weapon complex.

[2] This allowed missiles to be launched in a water depth of 40-60 m.[5] The propulsion system used an Inhibited red fuming nitric acid-amine propellants combination, AK-27I/TG-02.

[1] There was some confusion about the SS-N-4/5/6 series of missiles in the West, the SS-N-5 is normally given the NATO reporting name 'Sark' like the SS-N-4 first carried by the Golf submarines,[7] but some variants were assigned the name 'Serb' normally used for the SS-N-6.